Constructing a scanner with an extended light source such as an arc lamp or a high power laser diode or a laser diode array poses a difficult task for a designer. Because of the extended size of the source, it is impossible to collect all the light transmitted by the source into one or more small spots, for example of diameter on the order of 10 micrometers at a sufficiently high exposure level. Laser image recorders typically use lasers with a very well defined optical beam, for example, a low power laser diode, or a gas laser. Such laser diodes provide a high enough beam quality to be focused with high efficiency to a relatively small spot, and thus have been widely used in office laser printers and in the graphic arts industry.
For low sensitivity media printers such as a laser thermal printer, a higher power laser needs to be used to achieve a high throughput. CO.sub.2 and Nd-Yag lasers do provide high, clean power with nearly Gaussian beams, and can be focused onto small spots with high efficiency, but these are extremely expensive. In addition, a Nd-Yag laser has a wavelength of 1064 nm, while for thermal imaging a lower wavelength, typically 830 nm, is preferred. The high power is of particular importance when writing on thermal materials such as thermal offset plates. In this type of material, heat is used to cross-link polymer chains. The cross-linked polymer is later used as the ink attracting layer (the printing layer) in an offset press. The necessary energy may not be applied in a very short time period (less than 100 ns) since this would ablate the polymer layer. This is in contradiction with modern fast imaging systems that expose more than 50 million pixels per second, which corresponds to only 20 nanoseconds per pixel.
Another type of material to which the present invention is applicable is for exposing liquid materials. For example, in stereo lithography, a laser scans the surface of a tank filled with a liquid. Upon receiving the laser light, this liquid polymerizes to solid form. The solid layer is then lowered into the liquid until the solid is covered by the liquid. A new laser scan will harden the next layer of material, and so on. The object material is thus built up layer by layer. In the future, it is envisaged that a liquid might be applied to a plate prior to exposure. During exposure, this liquid would cure on the imaged parts. The cured and non-cured parts would then be used for printing. This process is similar to a thermal transfer process wherein a layer from a donor sheet is transferred to a plate.
It is the object of the invention to present an imaging apparatus that delivers the power of an extended line source with high efficiency to a row of small pixels in a fixed array on the recording material. An example of a suitable light source is a high power laser diode, for example one with power of more than about 1 W. Another example is a laser diode array. The imaging system is set up such that the energy delivered to each pixel, and the shape of each pixel in a fixed array on the recording material is essentially the same.
A second object of the invention is to provide a method of applying high-energy exposure doses in fast image scanning systems. The resulting imaging system can provide the energy in an approximately 5 .mu.s time interval despite the approximately 50 million pixels per second imaging rate.
A third object of the invention is to provide a means for altering the width of an imaged scan line. This enables changing the writing resolution of a scanning system without requiring sophisticated zoom optics. Such a variable resolution system enables higher imaging efficiency because the beam shape provided by the proposed method provides for a more rectangular profile. A rectangular profile is desirable when working with a binary material. With a Gaussian beam, only the center portion (that reaches the material threshold level) of the beam contributes to the image formation process. The portion of energy outside this center is lost.